This invention relates to a process for the separation of C.sub.2+ hydrocarbons from a feed stream of natural gas under pressure, and in particular wherein the natural gas is cooled, partially condensed, and then separated into a liquid fraction and a gaseous fraction.
It is common for separated liquid fraction to be subcooled (i.e., cooled below the boiling point of its components) and then expanded into the upper zone of a rectifying column. It is also common for the gaseous fraction, after being expanded within a turbine or engine, to be introduced into the rectifying column. The rectification or fractionating process within the column produces a product stream containing essentially C.sub.2+ hydrocarbons and a residual gas stream containing predominantly lower-boiling components, e.g., methane. It is also conventional, after being discharged from the rectifying column, for the residual gas to be reheated by heat exchange with the feed stream of natural gas.
In such an expansion method for C.sub.2+ separation from natural gas, the low temperatures required for attaining a high yield of C.sub.2+ hydrocarbons are produced by engine expansion of the gaseous fraction remaining after the partial condensation of the natural gas. During the expansion process, a large temperature difference occurs perforce between the inlet and outlet of the turbine or engine. The large temperature difference resulting from the cold-producing expansion is then effectively utilized in order to bring about an optimum increase in the yield of C.sub.2+ gas.
U.S. Pat. No. 4,140,504 describes a process of the type discussed above wherein a portion of the gaseous fraction remaining after partial condensation is not work expanded for production of refrigeration. Instead, this portion of the gaseous fraction is combined with the liquid fraction which results in a reduction of the bubble point of the liquid fraction. The combined stream is then cooled and condensed by heat exchange with process streams and expanded by an expansion device. During expansion, a portion of the stream vaporizes and this further cools the remaining portion. The expanded stream is then supplied as top feed to the rectifying column. This process has the drawback that the amount of the gaseous fraction which is engine-expanded in the expansion turbine, as well as the total resultant work produced by expansion of the gaseous fraction, is diminished.